Apparatus for improving stencil/screen print quality

ABSTRACT

A method and apparatus for improved stencil/screen print quality is disclosed. The stencil or screen assists in application of a printable material onto a substrate, such as an adhesive to a semiconductor die of a semiconductor wafer during a lead-on-chip (LOC) packaging process. In one embodiment, the stencil includes a coating applied to at least one surface of a pattern of the stencil or screen to retard running of the printable material onto the surface. In another embodiment, the stencil or screen includes a second coating applied to at least one other surface of the pattern to promote spreading of the printable material onto the substrate.

This application is a Divisional of U.S. Ser. No. 08/935,745 filed Sep.23, 1997.

FIELD OF THE INVENTION

This invention relates generally to stencil and screen prints, andparticularly to improving the quality of such stencil and screen prints.

BACKGROUND OF THE INVENTION

Stencils and screens to apply patterns or printable material onsubstrates have been used in many contexts. For example, such apatterned adhesive layer, as described in Farnworth et al., U.S. Pat.No. 5,286,697, which is hereby incorporated by reference, may be used toattach a semiconductor die to a lead frame. Farnworth et al. describesthe use of a patterned screen to deposit an adhesive layer on the die sothat the die can be mounted to the lead frame. The screen is patternedso that the streets between individual dies on a wafer are free fromadhesive to permit later sawing of the wafer into separate the dies, andso that the bond pads are also free fiom adhesive to permit laterconnection of the bond wires between the lead fingers of the lead frameand the bond pads.

Farnworth et al. specifically teaches a screen printing process in whichthe patterned screen is situated between the wafer and a liquid adhesivenozzle. Once the screen is properly aligned over the wafer, liquidadhesive is released from the nozzle, coating the wafer with adhesive inthe desired pattern. The screen is then removed from the wafer, and thedies of the wafer are attached to a lead frame and separated from oneanother. The application of liquid adhesive on a wafer using a screen asdescribed in Farnworth et al. is similar to a silk screen process usedfor printing artwork, such as on T-shirts. This same stencil and screenprinting process similar to the silk screen process used for printingartwork on T-shirts has also been utilized in other semiconductorapplications. Such semiconductor applications include printing apatterned nonconductive polyimide barrier for flip chips, printingconductive adhesive bumps or solder bumps, and printing conductive inkon flexible printed circuit boards (PCBs).

However, it has been found that a shortcoming to such screen and stencilprintings is the migration or overflow of the printable material (suchas the adhesive or the ink) underneath the bottom of the screen, whichcauses the substrate to which the printable material is being applied(such as the wafer or the printed circuit board) to stick to the screen.This results in a poorly patterned layer. More significantly, migrationunderneath the bottom of the screen shortens screen life, which isdefined as the number of consecutive uses, or prints, of the screenwithout cleaning of the screen. Migration underneath the bottom of thescreen forces cleaning of the screen between prints to ensureconsecutive clear prints. Reduced screen life therefore slows down thescreen and stencil printing process, and adds cost to the process.

SUMMARY OF THE INVENTION

The above-mentioned shortcomings are addressed by the present invention,which will be understood by reading and studying the followingspecification. The invention describes a stencil and screen used toassist in the application of a patterned printable material layer on asubstrate. For example, the stencil or screen may be used to apply anadhesive to a semiconductor die, which is typically a part of asemiconductor wafer. Preferably, the stencil or screen includes acoating applied to at least one surface of a patern to retard migrationor overflow of the printable material onto the surface. Alternatively,the stencil or screen includes a second coating applied to at least oneother surface of the pattern to promote spreading of the printablematerial onto the substrate to which it is being applied.

For example, the pattern may include a number of apertures. The sidewalls of these apertures are side surfaces of the pattern. During theprintable material application process, the stencil or screen is alignedover the substrate to which the printable material is to be applied. Thebottom surface of the screen or stencil is coated with a material suchas polytetrafloroethylene or another polymeric material to retardrunning of the printable material onto the bottom surface (i.e., betweenthe stencil or screen and the substrate). The top surface and sidesurfaces of the pattern are alternatively coated with a material such astungsten to promote spreading of the printable material from the top andside surfaces to the parts of the object surface exposed underneath.

The material coating the bottom surface of the pattern, which isdesirably a polymeric material, in accordance with an embodiment of theinvention, has three advantageous aspects. First, it acts as a lowsurface tension layer to minimize printable material migration in orderto provide better print definition. Second, it acts as a gasket layer toconstrain the flow of the printable material. Third, it acts as a bufferlayer to minimize the stencil height inconsistency caused by defects,such as burr, on the bottom of the stencil and the top surface of thesubstrate onto which the printable material is applied.

A stencil or screen according to an embodiment of the inventiontherefore increases life of the stencil or screen. That is, consecutiveprints can be made using a stencil or screen according to an embodimentof the invention without cleaning. This results in faster screenprinting and reduces cost of the printing. Still other and furtheradvantages, aspects and embodiments of the invention will becomeapparent by reference to the drawings and by reading the followingdetailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a prior art lead frame suitable for usewith a lead-on-chip (LOC) die;

FIG. 2 is a perspective view of a prior art package lead-on-chip (LOC)die;

FIG. 3 is a flow chart of a screen printing method for coating a waferand dies with a patterned adhesive layer using a stencil according toone embodiment of the invention;

FIG. 4 is a side view of the screen printing method of FIG. 3,illustrating the distinct layers of a stencil according to oneembodiment of the invention in detail;

FIG. 5(a) is a side view showing adhesive applied to a surface of astencil according to one embodiment of the invention having a highsurface tension; and,

FIG. 5(b) is a side view showing adhesive applied to a surface of astencil according to one embodiment of the invention having a lowsurface tension.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the appended claims.

The invention as it is described in conjunction with FIGS. 1-4, 5(a) and5(b) is described in relation to the application of an adhesive layeronto a semiconductor die so that the die may be attached to a leadframe. Those of ordinary skill within the art, however, will appreciatethat the invention is not limited to such an application. Anyapplication that may be benefit from improved stencil or screen printingin accordance with the invention is amenable to the invention. Forexample, the printing of artwork on T-shirts, the printing of patternednonconductive polyimide barriers for flip chips, the printing ofconductive adhesive bumps or solder bumps, and the printing ofconductive ink on flexible printed circuit board are also all within thescope of the invention.

As those of ordinary skill within the art will recognize, the termsstencil and screen are generally speaking quite similar to each other.However, the pattern within a given stencil generally has largerapertures than the pattern within a given screen. Furthermore, a screentypically includes an emulsion layer on both the top and bottom surfacesof its pattern. The invention is generally described herein relative toa stencil. However, those of ordinary skill within the art willappreciate that the invention is equally applicable to screens.

Referring first to FIG. 1, a representative prior art lead-on-chip (LOC)die 110 and lead frame 112 are shown. The LOC die 110 is thin, flat andgenerally rectangular in shape and includes a die face 114, or activeside, wherein the integrated circuitry is formed. A plurality of diewire bonding pads 116 are formed across the center and side edges of thedie 110 in electrical contact with the integrated circuits formed on thedie 110.

The lead frame 112 is formed from metal sheet and includes side rails118 and 120 formed with indexing holes 122. The indexing holes 122facilitate transport and indexing of the lead frame 112 by automatedpackaging machinery. The lead frame 112 also includes side bars 124 and126 for increased rigidity and to limit the flow of encapsulatingmaterial during the encapsulation process. The side rails 118 and 120and side bars 124 and 126 are trimmed away during a trim and formoperation.

Furthermore, the lead frame 112 includes a plurality of generallyparallel and spaced lead fingers 128. During the packaging process thelead fingers 128 are connected to thin bond wires 130 (as shown in FIG.2) which also connect to the wire bond pads 116. The lead fingers 128thus function to electrically connect the integrated circuitry formed onthe LOC die 110 to external circuitry (e.g., a printed circuit board).In addition, the lead fingers 128 function to support the LOC die 110during the packing process and to facilitate heat transfer away from LOCdie 110. The lead frame 112 also includes bus bars 132 for makingmultiple connections to the wire bond pads 116.

Referring next to FIG. 2, the packaged prior art LOC die 110 orsemiconductor package 134 is shown. The semiconductor package 134includes an encapsulating material 136 which encapsulates the chip 110and all but a terminal portion 138 of the lead fingers 128. Kinks 140 inthe lead fingers 128 help to rigidify the assembly. The encapsulatingmaterial 136 is typically formed of an insulative plastic material. TheLOC die 110 is located in the center of the package 134 with the wirebond pads 116 of the die 110 connected to the thin bond wires 130, whichin turn connect to the lead fingers 128.

In addition to being attached to the thin bond wires 130, the leadfingers 128 also attach to the front side 114 of the die 110, or to analpha barrier (not shown) attached to the die 110. Prior toencapsulation, the lead fingers 128 provide the only physical connectionbetween the lead frame 112 and the LOC die 110. As has been described inthe background, a process for attaching the lead fingers 128 to the die110 is to apply adhesive to the die 110 so that the die can then beaffixed to the lead fingers 128. The process by which adhesive isapplied to the die 10 so that the lead fingers 128 can be attachedthereto is shown by reference to FIG. 3 and FIG. 4.

Referring first to FIG. 3, a flowchart of a screen printing method forcoating a wafer and dies with a patterned adhesive layer using astencil, according to one embodiment of the invention, is shown. Theprocess of FIG. 3 is a hot screen printing process similar to a silkscreening process used for printing artwork. This is an exemplaryprocess by which a stencil according to an embodiment of the inventionmay be utilized. The invention is not so limited, however, and otherprocesses, such as a cold screen printing, are amenable to applicationof embodiments of the invention.

In a first step 210 of a hot screen printing process, a stencil isaligned over a wafer, which is one type of substrate onto whichprintable material may be applied. Other types of substrate includeprinted circuit boards (PCBs), T-shirts, silk, paper, die, and ceramics;the invention is not limited to any particular type of substrate,however. The wafer is patterned to correspond to a desired adhesivepattern on the wafer. The pattern of the screen is such that the streetsbetween individual dies on the wafer are free from adhesive to permitlater sawing of the wafer to separate the dies, and so that the bondpads of the dies are also free from adhesive to permit later connectionof the bond wires between the lead fingers of the lead frame and thebond pads.

In step 212, adhesive is sprayed over the stencil. The adhesive is onetype of printable material which can be applied to a substrate. Othertypes of printable material include ink, polymer resins, diluted orsoluble polymers, composite materials, and solder paste; the inventionis not limited to any particular type of printable material, however.Adhesive flows through the apertures of the stencil, and onto the wafer.The hot screen printing process is preferably performed at an elevatedtemperature of about 100° C. to about 500° C., to facilitate theflowability of the adhesive.

In step 214, the adhesive-coated wafer is removed from underneath thestencil, and the packaging process as has been already generallydescribed continues with respect to that wafer. The wafer is separatedinto individual dies, which are then each attached to lead fingers of alead frame via the adhesive coating on the dies. The dies are ultimatelyencapsulated in a protective package as has been described inconjunction with FIG. 2. After encapsulation, the resulting integratedcircuit chips are tested for proper performance, after which they areready for inclusion into an electronic device such as a computer.

If there are no more wafers to which adhesive needs to be applied,control proceeds from step 216 to step 218, and the hot screen printingprocess is finished. Otherwise, the stencil is checked for cleanlinessin step 220 by visual inspection. The adhesive, when sprayed over thestencil for application onto the wafer, has a tendency to migratebetween the stencil and the wafer, and stick to the underside of thestencil. An advantage of the inventive stencil described herein is thatit does not always require cleaning between immediately subsequent uses.Thus, if the stencil is still clean, control proceeds from step 220 backto step 210, where it is aligned over a new wafer, and the hot screenprinting process of FIG. 3 starts over. Otherwise, the stencil is firstcleaned in step 222 before the process starts over at 210 with respectto a new wafer.

Referring next to FIG. 4, a side view illustrating the screen printingmethod of FIG. 3 is shown. In a hot screen printing process,semiconductor wafer 10, which usually includes a plurality ofsemiconductor dies, is coated with a liquid adhesive layer 12 dispensedfrom a nozzle 14. A stencil 16 having a pattern is situated between thewafer 10 and the nozzle 14 to provide the desired pattern. As shown inFIG. 4, stencil 16 includes apertures corresponding to the desiredadhesive layer pattern. Only one aperture is shown for simplicity andclarity of description and illustration. What is referred to as sidesurfaces of pattern 16 includes, but is not necessarily limited to, theside walls of the apertures.

A dockering roller 18 (or blade), moveably mounted for motion across thestencil 16 as indicated by arrow 20, can be used to aid in thedistribution of the adhesive. The adhesive is a suitable thermoplasticor thermoset adhesive. Such suitable thermoplastic adhesive materialsinclude those sold under the trade name Ablestik XR-041395, availablefrom Ablestik, Inc., and which is a polyimide. Such suitable thermosetadhesives include those sold under the trade name Ablestik 8176T, alsoavailable from Ablestik, Inc, and which is a phenolic resin.

The pattern of stencil 16 has one or more surfaces modified inaccordance to an embodiment of the invention. For example, it has beenfound that adhesive tends to migrate onto the bottom surface of thepattern, between stencil 16 and wafer 10, during the applicationprocess. Therefore, preferably the bottom surface has a coating 24applied thereto to retard running of the adhesive onto the surface.Furthermore, preferably side surfaces of the pattern (i.e., betweenwhich adhesive 12 is deposited), as well as the top surface of thepattern, have a coating (or plating) 26 applied thereto to promotespreading of the adhesive onto the wafer.

The stencil 16 having its surfaces so coated has an extended life, andtherefore can be used for consecutive prints without cleaning.Therefore, in an exemplary method of the invention, the stencil isaligned over a first semiconductor wafer, adhesive is sprayed over thestencil to form a patterned adhesive layer on the wafer, and then thestencil is aligned over a second wafer, and adhesive is sprayed a secondtime over the stencil to form an adhesive layer on the second wafer. Thestencil does not require cleaning after the spraying of the adhesiveover the stencil when it is aligned over the first wafer, and before thealigning of the stencil over the second wafer.

With respect to a preferred construction of pattern 16, the patternitself is preferably constructed from metals or metal alloys. One suchmetal alloy is stainless steel. Stainless steel stencils are commonlyavailable commercially, (e.g., from Photo Stencil, Inc., of ColoradoSprings, Colo., and from UTZ Engineering, Inc., of Clifton, N.J.).Stainless steel stencils in particular have a surface tension of 1384dyne/cm at melting point.

Coating 24 is applied underneath the bottom surface of pattern 16 toretard running of the adhesive onto the bottom surface. Preferably,coating 24 has a surface tension less than the surface tension ofpattern 16. One such coating is polytetrafloroethylene, which is apolymeric material available under the trade name DuPont Teflon,available from DuPont, Inc. Polytetrafloroethylene has a criticalsurface tension of 23.9 dyne/cm at room temperature. Other polymers mayalso be used since surface tension of most polymers is one order lessthan that of metals or ceramics. Because its surface tension is markedlyless than that of pattern 16, the running property of the adhesive ontothe bottom surface of pattern 16 is controlled. Pattern 16 is initiallyconstructed from a metal or metal alloy having a high surface tension tospread the adhesive onto the wafer.

Furthermore, to promote the spreading of the adhesive on thesemiconductor wafer, plating 26 is applied to at least the side surfacesor the top surface of pattern 16. As shown in FIG. 4, plating 26 isapplied to both the top surface and the side surfaces of pattern 16.Plating 26 is a coating having a surface tension greater than thesurface tension of pattern 16. One such plating is tungsten, which has asurface tension of 2500 dyne/cm at melting point. Other platings havingsimilarly high surface tensions include tungsten carbide, and tungstennitride. Still other platings having high surface tensions includenickel and nickel alloy. Because the surface tension of plating 26 isgreater than that of pattern 16, the spreading of the adhesive onto thesemiconductor wafer is promoted.

Those skilled within the art will recognize that the embodiment shown inFIG. 4 is only a preferred embodiment of the invention. Otherembodiments conforming to the basic principles of the invention are alsocontemplated. For example, a stencil having a pattern with a coating onthe bottom surface of the pattern to retard running of adhesive onto thesurface, but not necessarily having a plating or second coating on thetop or side surfaces of the pattern to promote spreading of the adhesiveonto the semiconductor die, is within the scope of the invention. Forfurther example, a stencil having a pattern with a plating or coating onat least either the top or side surfaces of the pattern to promotespreading of adhesive onto the die, but without a coating on the bottomsurface of the pattern to retard running of the adhesive onto the bottomsurface, is also within the scope of the invention.

As has been described, preferably coating 24 has a surface tension lessthan that of pattern 16, and plating 26 has a surface tension greaterthan that of pattern 16. This permits the coating to retard running ofthe adhesive, and the plating to promote spreading of the adhesive ontothe semiconductor wafer. The reason why the coating and the platingperform these functions in the preferred embodiment is shown byreference to FIG. 5(a) and FIG. 5(b). Coating 24, plating 26 as appliedto the top surface of the pattern, and pattern 16, can be referred to asseparate layers of the semiconductor die stencil shown in FIG. 4.

Referring first to FIG. 5(a), surface 28 is a surface having a highsurface tension, such as that of plating 26 of FIG. 4. Because thesurface tension is high, surface 28 tends to pull adhesive 30 across theentire surface. The greater the surface tension, the more the adhesive30 is spread across surface 28. Therefore, while pattern 16 of FIG. 4preferably has a sufficient high surface tension (1384 dyne/cm) tospread the adhesive, the addition of plating 26, which has an evengreater surface tension (2500 dyne/cm), promotes the spreading of theadhesive even more.

Referring next to FIG. 5(b), surface 32 is a surface having a lowsurface tension, such as that of coating 24 of FIG. 4. Because thesurface tension is low, surface 32 does not tend to pull adhesive 34across the entire surface. The adhesive remains where it first islocated, and does not migrate or run across the surface. The lower thesurface tension, the less the adhesive 34 tends to be pulled acrosssurface 32. Thus, because preferably coating 24 of FIG. 4 has a muchlower surface tension than pattern 16 (23.9 dyne/cm compared to 1384dyne/cm), when the adhesive reaches an edge of coating 24, coating 24retards the adhesive from running or migrating any further.

A surface-modified stencil has been described. Although specificembodiments have been illustrated and described herein, it will beappreciated by those of ordinary skill in the art that any arrangementwhich is calculated to achieve the same purpose may be substituted forthe specific embodiments shown. This application is intended to coverany adaptations or variations of the present invention. Furthermore,while the invention has been described specifically in the context ofthe application of adhesive to a semiconductor die, the invention is notlimited to such an end use, and the invention is amenable to othersemiconductor and non-semiconductor uses as well. Therefore, it ismanifestly intended that this invention be limited only by the followingclaims and equivalents thereof.

We claim:
 1. A semiconductor die stencil to assist in application of aprintable adhesive material onto a substrate comprising: a pattern; acoating applied to at least one surface of the pattern to retardspreading of the printable adhesive material onto the at least onesurface; and wherein the coating is applied to a bottom surface of thepattern, and the stencil further comprises a second coating applied toat least one other surface of the pattern to promote spreading of theprintable adhesive material.
 2. The semiconductor die stencil of claim1, wherein the coating is a polymeric coating.
 3. The semiconductor diestencil of claim 1, wherein the second coating is selected from thegroup consisting of tungsten, tungsten carbide, tungsten nitride,nickel, and nickel alloy.
 4. A semiconductor die stencil to assist inapplication of a printable adhesive material onto a substrate,comprising: a sheet of material; a plurality of apertures in the sheetof material defining a desired pattern; a polymeric coating applied to afirst surface of the sheet of material to retard spreading of theprintable adhesive material; and a second coating applied to a secondsurface of the sheet of material to promote spreading of the printableadhesive material.
 5. The semiconductor die stencil of claim 4, whereinthe second coating is selected from the group consisting of tungsten,tungsten carbide, tungsten nitride, nickel, and nickel alloy.
 6. Asemiconductor die stencil to assist in application of a printableadhesive material onto a substrate, comprising: a sheet of material; aplurality of apertures in the sheet of material defining a desiredpattern; a polymeric coating having a surface tension which is less thana surface tension of the sheet of material whereby the polymeric coatingis applied to a first surface of the sheet of material to retardspreading of the printable adhesive material; and a second coatinghaving a surface tension which is greater than the surface tension ofthe sheet of material whereby the second coating is applied to a secondsurface of the sheet of material to promote spreading of the printableadhesive material.
 7. The semiconductor die stencil of claim 6, whereinthe second coating is selected from the group consisting of tungsten,tungsten carbide, tungsten nitride, nickel, and nickel alloy.
 8. Asemiconductor die stencil to assist in application of a printableadhesive material onto a substrate, comprising: a sheet of materialcomprised of a metal or metal alloy; a plurality of apertures in thesheet of material defining a desired pattern; a first coating applied atop surface of the sheet of material and to side walls of the apertureswhere the first coating promotes spreading of the printable adhesivematerial across the top surface and into the apertures; and a secondcoating applied to a bottom surface of the sheet of material, where thesecond material retards migration of printable adhesive material acrossthe bottom surface.
 9. The semiconductor die stencil of claim 8, whereinthe first coating is selected from the group consisting of tungsten,tungsten carbide, tungsten nitride, nickel, and nickel alloy.
 10. Asemiconductor die stencil to assist in application of a printablematerial onto a substrate, comprising: a sheet of metal; aperturesthrough the sheet of metal defining a pattern; a first coating having afirst surface tension applied a top surface of the sheet of metal and toside walls of the apertures where the first coating has a surfacetension selected to promote spreading of printable material across thetop surface and into the apertures; and a second coating having a secondsurface tension applied to a bottom surface of the sheet of material,where the second material has a surface tension selected to retardmigration of printable material across the bottom surface.
 11. Thesemiconductor die stencil of claim 10, wherein the first coating isselected from the group consisting of tungsten, tungsten carbide,tungsten nitride, nickel, and nickel alloy.
 12. The semiconductor diestencil of claim 10, wherein the sheet of metal is stainless steel. 13.A semiconductor die stencil to assist application of a printableadhesive material onto a substrate, comprising: a sheet of metal ormetal alloy; apertures in the sheet defining a pattern; a metalliccoating having a first surface tension applied the top surface of thesheet and to side walls of the apertures to promote spreading of theprintable adhesive material; and a polymeric coating having a secondsurface tension applied to the bottom surface of the sheet to retardspreading of the printable adhesive material.
 14. The semiconductor diestencil of claim 13, wherein the metallic coating is selected from thegroup consisting of tungsten, tungsten carbide, tungsten nitride,nickel, and nickel alloy.
 15. The semiconductor die stencil of claim 13,wherein the sheet of metal or metal alloy is stainless steel.